The present invention relates to a lubricant for a magnetic recording medium, a magnetic recording medium, and a method for making the magnetic recording medium, used with an external recording device of a computer. In particular, the present invention improves lubrication quality and CSS durability of a magnetic recording medium by coating the surface of the medium with a liquid lubricant.
Recording devices and the like using solid magnetic recording media generally use a contact start stop (CSS) system in which a magnetic head is levitated while the magnetic recording medium is rotating, and when the rotation driving motor is stopped, the magnetic head makes contact with the magnetic recording medium. In the magnetic recording medium of the prior art, diamond-like carbon (DLC) is used as a surface protection film, with a small amount of N or Si and the like. In general, in order to improve the lubrication quality of this surface protection layer, a perfluoropolyether lubricant is coated on top of the surface protection layer, either alone or with a cyclic triphosphazene lubricant. The perfluoropolyether lubricant has a polar end group, such as a hydroxyl group or a piperonyl group. Adsorption of gases and organic contamination onto the surface of the recording media is prevented by completely covering the protection layer surface with this lubricant. Using the lubricant improves lubrication quality and provides a magnetic recording medium that is stable and has excellent CSS durability.
The DLC carbon surface is covered by a thin oxidation film, having a reactive carbonyl, carboxyl, hydroxyl, or other similar functional group. Pollutants are readily adsorbed and bound to this oxidation film. It is desirable to suppress this sorption of contaminants. This can be accomplished by using a perfluoropolyether with a polar end group. However, if the molecular weight of the perfluoropolyether is too low, lubrication quality is reduced. If it is too high, the sorption tendency increases. In general, a perflouropolyether having an average molecular weight of 1,500-5,500 is used. In order to completely cover the protective layer surface using this perfluoropolyether, the lubricant must be at least 50 xc3x85 thick. However, when a perfluoropolyether with a relatively large molecular weight is used, coating films only several tens of xc3x85""s thick on the protective layer result in there being spaces between molecules, making it difficult to completely cover the protective layer surface. As a result, providing a sufficiently thick coating film, while avoiding attachment between the magnetic recording medium and the magnetic head, becomes difficult.
In recent years, in addition to development of high density recording, advances have been made in development of low levitation magnetic heads. Low levitation heads now being made, such as the Tri-Pad head, the MR head and the like, are gradually replacing the TRC heads of the past. As a result of using low levitation heads, degradation of the main chain area (ether region) of the perfluoropolyether lubricant occurs, due to either catalytic action from the head material or from friction heat generated during use of the recording device. Furthermore, corrosive components, such as degradation products and gases that have been adsorbed to the disk surface, are transferred to the magnetic head surface. This disrupts the levitation quality of the magnetic head, and invites reduction in replay output. Furthermore, once the perfluoropolyether lubricant is degraded, there is a reduction in the lubrication quality, and abrasion in the protection layer occurs. Under the worst case scenario, head crushing can result from such abrasion.
It is known from the prior art that cyclic triphosphazene lubricants improve lubrication quality and have the effect of suppressing the degradation of perfluoropolyether lubricants. For example, Japanese Laid-Open Patent Publication Number 9-305961, discloses a combination of a cyclic triphosphazene lubricant and a perfluoropolyether lubricant. The cyclic triphosphazene lubricant is represented by the structural formula (III) below. When this combination is used, the lubrication quality of the magnetic recording medium is improved. 
In formula (III), n is 0 or an integer from 1 to 6.
However, certain problems remain to be solved. For example, perfluoropolyether lubricants and cyclic triphosphazene lubricants are very difficult to mix. When they are used as a lubricating layer film, aggregation can occur under certain conditions. As a result, the quality of the recording device using a fixed magnetic recording medium can be compromised when using the prior art lubricant.
In order to solve the aforementioned problems, various types of perfluoropolyether lubricants have been tried. To date, a technology having a satisfactory overall performance has yet to be achieved.
In light of the above, it is an object of the present invention to overcome the limitations of the prior art.
An object of the present invention is to provide a lubricant for a magnetic recording medium that avoids degradation of the lubricant and abrasion of the protection layer caused by low levitation of the magnetic head in high density recording.
A further object is to provide a magnetic recording medium having stable lubricant quality (improvement of CSS durability) over the long term.
A further object is to provide a method for making a magnetic recording medium, where the medium possesses stable lubricant quality.
In order to achieve the above objectives, the present inventors have conducted intensive study into a lubricant used in a lubrication layer. As a result, a compound having a cyclic triphosphazene lubricant and a perfluoropolyether lubricant in a single molecule was discovered. The composition of the present invention improves lubrication quality and is highly effective in suppressing degradation of the perfluoropolyether lubricant.
Briefly stated, the present invention is a lubricant for a magnetic recording medium that imparts resistance to degradation, improved lubrication quality and better CSS durability to the medium, even when using a low levitating head. The lubricant is a perfluoropolyether compound having a cyclic triphosphazene end group within the molecule.
The above perfluoropolyether lubricant has a general structural formula (I): 
wherein n is an integer from 1 to 5, o is an integer from 1 to 5, provided that n+oxe2x89xa66. Rf is selected from the group consisting of:
HOxe2x80x94CH2CF2xe2x80x94Oxe2x80x94(xe2x80x94CF2xe2x80x94Oxe2x80x94)qxe2x80x94(CF2CF2xe2x80x94Oxe2x80x94)pxe2x80x94CF2CH2xe2x80x94,
F3Cxe2x80x94Oxe2x80x94(xe2x80x94CF2xe2x80x94Oxe2x80x94)qxe2x80x94(CF2CF2xe2x80x94Oxe2x80x94)pxe2x80x94CF2CH2xe2x80x94, and F(CF2CF2)3CH2xe2x80x94,
wherein p is 0 or an integer of 1 or greater and q is 0 or an integer of 1 or greater.
The perfluoropolyether lubricant of the present invention can be formed by a dip coat method, a spin coat method, and the like.
According to an embodiment of the invention, a magnetic recording medium is provided which uses a perfluoropolyether compound having a cyclic triphosphazene end group within the molecule as a lubricant. The magnetic recording medium has the sequential layers of a foundation layer, a magnetic layer, a protection layer, and a lubrication layer on a non-magnetic base. The lubrication layer includes at least one perfluoropolyether lubricant having a cyclic triphosphazene end group within the molecule.
According to another embodiment of the invention, a method of making a magnetic recording medium which resists degradation when using a low levitation head, is provided, including the sequential steps of: forming a foundation layer on a non-magnetic base, forming a magnetic layer on the foundation layer, forming a protection layer on the magnetic layer, forming a lubrication layer on the protection layer. The lubrication layer includes at least one perfluoropolyether lubricant having a cyclic triphosphazene end group within the molecule.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.